This invention relates to the production of positive working lithographic plates.
Positive working lithographic plates are very widely manufactured and probably account for over half of all lithographic plates. The photosensitive compositions used are comparatively simple and easy to manufacture, typically comprising an o-quinonediazide light sensitive component and a phenolic resin. Their processing after image-wise exposure is also simple. However, if the processing is not carried out according to the manufacturers instructions, such as the use of developing solutions which are stronger than that recommended or if temperature of the developing solution is too hot, there is a tendency for the developing solution to remove not only the exposed areas of the plate but also to attack the image areas in the non-exposed areas of the plate. This attack on the printing image areas produces a poor image and in many cases renders the plate unusable. The use of strong developing solutions exacerbates the problem of image attack, as does developing times over 30 seconds as are often used in the processing of direct positive plates.
Common formulation means for decreasing attack by strong developers or through the use of conditions leading to over development, such as using phenolic resins with reduced alkali solubility, would normally lead to a reduction in plate sensitivity for the same plate under less aggressive development conditions. Accordingly methods to increase plate sensitivities, such as the use of speed enhancers, often result in reduced resistance to developer attack
We have discovered a positive working lithographic plate precursor and methods of producing said precursor which exhibit a reduced tendency to developer attack of the image obtained after image-wise exposure under aggressive development whilst significantly reducing any accompanying loss of sensitivity in less aggressive developers and development conditions.
Therefore according to the present invention there is provided a positive working lithographic plate precursor comprising a support having a hydrophilic surface and a coating thereon comprising a positive working photosensitive composition and an aryl alkyl polysiloxane.
According to a further aspect of the invention there is provided a positive working lithographic plate precursor comprising a support having a hydrophilic surface and a coating thereon comprising a positive working, photosensitive composition and an aryl alkyl polysiloxane at least part of which aryl alkyl polysiloxane is present as a thin layer at the uppermost surface of the plate precursor.
According to a further aspect of the present invention there is provided a method of preparing a positive working lithographic plate precursor of the present invention comprising coating on a support having a hydrophilic surface a coating comprising in admixture a positive working photosensitive composition and an aryl alkyl polysiloxane.
A preferred aryl alkyl polysiloxane useful in the present invention is a phenyl methyl polysiloxane.
Suitably the aryl alkyl polysiloxane constitutes at least 0.3%, preferably at least 1%, preferably up to 50%, more preferably up to 10% by weight of the dry coating. Thus a preferred weight range for the aryl alkyl polysiloxane may be expressed as 1-10% of the total dry coating weight.
The preferred positive working photosensitive compositions of the present invention are those comprising an o-napthoquinone diazide as the light sensitive component. Most preferably these are present either as sulphonic acid esters or as carboxylic acid esters.
Examples of particularly preferred o-quinone diazide compounds are disclosed in a variety of publications such as U.S. Pat. Nos. 2,766,118; 2,767,092, 2,772,972; 2,859,112; 2,907,665; 3,046,110; 3,046,111; 3,046,115; 3,046,118; 3,046,119; 3,046,120; 3,046,121; 3,046,122, 3,046,123; 3,061,430; 3,102,809, 3,106,465; 3,635,709 and 3,647,443 and these compounds may preferably be used in the invention. Among these, particularly preferred are o-naphthoquinonediazidosulfonates or o-naphthoquinonediazido carboxylates of aromatic hydroxyl compounds; o-naphthoquinonediazidosulfonic acid amides or o-naphthoquinonediazido-carboxylic acid amides of aromatic amine compounds, for instance, esters of benzoquinone-1,2-diazidosulfonic acid or naphthoquinone-1,2-diazidosulfonic acid with polyhydroxyphenyl (hereinafter the term xe2x80x9cesterxe2x80x9d also include partial esters); esters of naphthoquinone-1,2-diazido-4-sulfonic acid or naphthoquinone-1,2-diazido-5-sulfonic acid with pyrogallo/acetone, resins; esters of benzoquinone-1,2- diazidosulfonic acid or naphthoquinone-1,2-diazidosulfonic acid with novolak type phenol/formaldehyde resins or novalak type cresol/formaldehyde resins; amides of poly(p-aminostyrene) and naphthoquinone-1,2-diazido4-sulfonic acid or naphthoquinone-1,2- diazido-5-sulfonic acid; esters of poly(p-hydroxystyrene) and naphthoquinone-1,2- diazido-5-sulfonic acid; esters of polyethylene glycol with naphthoquinone-1,2-diazido-4-sulfonic acid or naphthoquinone-1, 2-diazido 5-sulfonic acid: amides of polymeric amines with naphthoquinone-1,2-diazido-4-sulfonic.
The preferred binder for the photosensitive composition is an alkali soluble resin. Particularly useful as the alkali soluble resins are the condensation products from the interaction between phenol, C-alkyl substituted phenols (such as cresols and p-tert-butyl-phenol), diphenols (such as bisphenol-A) and aldehydes (such as formaldehyde). Particularly useful in this invention are novolak resins, resole resins and novolak/resole resin mixtures.
The most preferred photosensitive composition comprises o-napthoquinonediazide sulphonyl esters of a phenolic resin.
Suitably the photosensitive composition constitutes at least 50%, preferably at least 90%, preferably up to 99.7%, more preferably up to 99% by weight of the dry coating. Thus a preferred weight range for the photosensitive composition may be expressed as 90-99% of the total dry coating weight.
Suitable solvents for the coating solutions are, for example, the known glycol derivatives (such as 2-ethoxy ethanol and 1-methoxy-propan-2-ol) and ketones (such as butanone) in common use for the manufacture of such photosensitive coatings.
The wet coated composition is dried at elevated temperatures to remove coating solvents. If drying temperature or drying time is too low then residual solvents can adversely effect the resulting developer resistance of the coating. If drying, is too severe then unwanted reactions can occur in the drying film. Preferably the coated compositions are dried between 100xc2x0 C. for 210 seconds and 130xc2x0 C. for 90 seconds.
According to a further preferred embodiment there is provided a method of preparing a positive working lithographic plate precursor according to the present invention comprising coating on a support having a hydrophilic surface a positive working photosensitive composition and applying an additional composition comprising an aryl alkyl polysiloxane on said photosensitive layer.
Suitably the aryl alkyl polysiloxane is laid down to a dry film weight of at least 0.02 gmxe2x88x922, preferably at least 0.05 gmxe2x88x922, preferably up to 0.6 gmxe2x88x922, more preferably up to 0.4 gmxe2x88x922. Thus a preferred weight range for the aryl alkyl polysiloxane may be expressed as 0.05-0.4 gmxe2x88x922 dry film weight.
The aryl alkyl polysiloxane composition can be applied to the photosensitive layer by various means such as spraying. Suitably the composition is coated onto the photosensitive layer.
Whilst the applicants do not wish to be limited by any theoretical explanation of how their invention operates, it is believed that the presence of at least part of the aryl alkyl polysiloxane at the uppermost surface of the precursor is a key factor. It is believed that during the first method of the present invention at least part of the aryl alkyl polysiloxane separates out of the coating and deposits thereon. Thus resistance to developer attack is manifested particularly at the surface of the coating in all precursors of the present invention. Dynamic contact angle studies (using a Cahn Dynamic Contact Angle Analyser) have clearly showed a marked effect on the surface of plates prepared according to the methods of the present invention. A typical positive working lithographic printing, plate precursor has advancing and receding contact angles in water of approximately 95xc2x0 and 48xc2x0 respectively. Whereas a precursor provided by either method of the present invention comprising, for example, a phenyl methyl polysiloxane has advancing and receding contact angles in water of approximately 95xc2x0 and 67xc2x0 respectively. A surface of the same phenyl methyl polysiloxane alone provided on the same substrate has advancing and receding contact angles in water of approximately 95xc2x0 and 67xc2x0 respectively. Thus a surface change has been affected on the precursor which provides a surface nature similar to that of the polysiloxane coated as a single component.
The base which can be used as the support having a hydrophilic surface is preferably an aluminium plate which has undergone the usual anodic, graining and post-anodic treatments well known in the lithographic art for enabling a photosensitive composition to be coated thereon and for the surface of the support to function as a printing background.
Another base material which may be used in the method of the present invention is a plastic material base or a treated paper base as used in the photographic industry. A particularly useful plastic material base is polyethylene terephthalate which has been subbed to render its surface hydrophilic. Also a so-called resin coated paper which has been corona discharge treated can also be used.
The photosensitive compositions of the invention may contain other ingredients such as stabilizing additives, colorants, color change additives, additional polymeric binders etc. as are present in many lithographic plate compositions.
The following Examples more particularly serve to illustrate various embodiments of the present invention described hereinabove.